Quick answer: A simple estimate is about 33 ml of water per kilogram of body weight each day, plus about 350 ml for every 30 minutes of exercise and extra in hot weather. A 70 kg person who exercises for 30 minutes a day in a mild climate needs about 2.7 liters of fluids.
There is no single number that fits everyone, but there are sensible starting points. Your body weight, how much you sweat during exercise, and the weather all change how much fluid you lose and need to replace.
This calculator starts from body weight and adds allowances for exercise and heat. The result covers all fluids, so tea, coffee and other drinks count toward it, and water from food helps too. Thirst and the color of your urine are good everyday guides: pale yellow usually means you are well hydrated.
How the estimate is made
Exercise = 0.35 liters for every 30 minutes
Hot or humid weather = + 0.5 liters
For comparison, the US National Academies set adequate total water intake (from all drinks and food) at about 3.7 liters a day for adult men and 2.7 liters for adult women. Roughly 20% of that normally comes from food, so your drinking target is lower than the total.
Needs increase during pregnancy and breastfeeding, in illness with fever, vomiting or diarrhoea, and at high altitude. People with kidney, heart or liver conditions, or on fluid restrictions, should follow their doctor's advice instead.
A worked example
Example. Someone who weighs 70 kg, exercises 30 minutes a day in a mild climate needs about 0.033 × 70 + 0.35 = 2.66 liters of fluid a day, roughly 11 glasses of 250 ml.
Real-world example
A 70 kg office worker needs about 70 × 33 = 2,310 ml, or 2.3 liters. On a day with a 30-minute run, add 350 ml for 2.7 liters in total. In a hot, humid climate the tool adds a further 500 ml. The National Academies suggest about 3.7 liters of total water a day for adult men and 2.7 liters for women, including the water in food, which is close to what this estimate gives.
Common mistakes to avoid
- Following “8 glasses a day” as a rule. It is a rough guide, and needs vary with body size, activity, weather and health.
- Counting only plain water. Tea, coffee, milk, juice and the water in food all count, and food supplies roughly a fifth of most people’s intake.
- Ignoring thirst and urine color. Pale yellow urine and normal thirst are good everyday signs that you are drinking enough.
Frequently asked questions
Do coffee and tea count as water?
Yes, mostly. Drinks with caffeine still contribute to your fluid intake for most people who drink them regularly, although plain water is a good default.
Can you drink too much water?
Yes, though it is uncommon. Drinking very large amounts in a short time can dilute sodium in the blood, a condition called hyponatremia. Sip steadily through the day rather than forcing large volumes.
Is the 8 glasses a day rule correct?
It is an easy-to-remember guideline, not a scientific requirement. Real needs vary with body size, activity and climate, which is why this calculator adjusts for them.
How do I know if I am drinking enough?
Pale yellow urine and rarely feeling thirsty are good signs. Dark urine, headaches, dry mouth or unusual tiredness can point to needing more fluid.
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Sources and further reading
- National Academies of Sciences, Engineering, and Medicine. “Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate.” 2005
- WHO: Obesity and overweight (fact sheet)
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